Investigating how combined multifidus injury and facet joint compression influence changes in surrounding muscles and facet degeneration in the rat

To examine whether unilateral multifidus damage could promote degeneration at the L5-6 facet joint (FJ) and compensatory changes in lumbo-pelvic muscles in rats.12 facet clamp, 12 facet sham and 7 control rats were studied. Facet clamp and sham animals had the left L5-6 FJ exposed, and the clamp group had a mild compressive clamp applied using hemostatic forceps to model post-traumatic arthritis. Both groups then had the left multifidus detached from the L1-L6 spinous processes. Animals were euthanized 28 days post-surgery. Muscle mass and fascicle length were evaluated bilaterally for the paraspinal muscles, gluteal muscles and biceps femoris. Intra-muscular collagen of the paraspinal muscles was measured histologically. FJ transverse plane angles were measured from micro-computed tomography scans. L5-6 FJ degeneration was evaluated through the 24-point OARSI scale.Differences, compared to control, were observed in the detached multifidus from both facet clamp and sham groups; namely decreased mass and fascicle length and increased collagen content. However, no between group differences were found for any other muscle. Further, mild FJ degeneration was more prevalent in the groups that had experienced multifidus injury but was not exacerbated by the mild compressive clamping of the FJ.Unilateral multifidus injury with or without FJ compressive clamping does not have a clear impact on the characteristics of surrounding spinal musculature within 28 days post-surgery in rats. Mild FJ degeneration was present in some animals from all three groups, and the impact of multifidus injury on this degeneration is inconclusive.